1. Field of the Invention
The present invention relates to a technique of carrying out communications between communication terminals on different local area networks (LANs), through a wide area network (WAN).
2. Description of Related Art
Use of a call control protocol such as a SIP (Session Initiation Protocol) or a DDNS (Dynamic Domain Name System) allows communication terminals on different LANs to directly carry out communications with each other through a WAN. Such a technique is referred to as a so-called VPN (Virtual Private Network). According to this technique, different remote LANs can be used as a directly-connected network.
Use of the technique allows, for example, connection between a LAN in a head office and a LAN in a branch office, so that terminals on the different LANs can carry out communications with each other through the Internet.
There has been known a technique capable of carrying out communications between communication terminals on different LANs, through the Internet. According to this technique, the different communication terminals on the different LANs log in to relay servers connected to the Internet via gateways so as to establish communication paths to the relay servers, respectively. Use of the communication paths allows the communication terminals to carry out communications with each other through the Internet. This technique allows communications between LANs via relay servers on a WAN.
In a case of using a SIP, typically, account information of each communication terminal is registered in a SIP-server. More specifically, when each communication terminal issues a REGISTER method to the SIP-server, the account information is registered in the SIP-server. Then, if one communication terminal makes a communication request which designates an account of another communication terminal, the SIP-server relays communications for establishment of a session in accordance with the registered account information.
If communications are carried out among plural LANs using the SIP, plural pieces of account information are registered in the SIP-server, leading to an increase in load imposed on the server. In order to carry out communications among plural LANs connected to one another, conventionally, accounts of all the LANs have been registered in a server which collectively controls the LANs. Consequently, a large load has been imposed on the server.
Moreover, each of the communication terminals needs to recognize an account of another communication terminal on a separate LAN in order to carry out communications with the other communication terminal. When the respective communication terminals access the server to acquire such account information, the load imposed on the server becomes larger. Further, since the server registers therein the accounts of the communication terminals on the plural LANs, each communication terminal requires a mechanism for readily retrieving the account of a target communication terminal from among the plural accounts.
In a case that the server collectively controls the account information, additionally, the account information of the communication terminals on all the LANs must be made unique. In other words, each LAN cannot obtain an account freely. Consequently, a degree of freedom inconveniently becomes small in terms of network control.